The present invention relates generally to Digital Subscriber Loop (DSL) technology and specifically to a method for masking crosstalk from adjacent loops.
Remote access and retrieval of data is becoming increasingly popular in data communication. The proliferation of the Internet has provided a vast network of information that is available to the general public. As the Internet grows and technology advances, this information is becoming increasingly voluminous and the details are become increasingly intricate. What used to comprise mainly of text information has grown to include still and moving images as well as sound. The increase in volume of information to be transferred has presented a need for a high-speed Internet connection, since traditional telephone modems communicate at speeds too slow for efficient communication.
One proposal for high-speed communication is the introduction of Digital Subscriber Line (DSL) technology. Currently, there many different DSL standards, including Asymmetric DSL (ADSL), High-speed DSL (HDSL), Very High Speed DSL (VDSL), Single-line DSL (SDSL), Single-line, High-speed DSL (SHDSL) and Integrated Services Digital Network (ISDN) DSL systems. Generically, the term xDSL is used to represent these, and other, standards. One of the most attractive features of xDSL is that it is implemented using an infrastructure that already exists. xDSL shares copper twisted pair lines typically used for telephone communication.
Some DSL technologies, including SDSL, ISDN DSL, SHDDL, and HDSL are baseband schemes that cover a band (0 to 4 kHz) dedicated to Plain Old Telephone Service (POTS). Thus, these schemes cannot co-exist with POTS. However, other DSL technologies, including ADSL and VDSL, share the twisted pair with POTS. However, only a small portion of the available bandwidth of the twisted pair line is used for POTS. These schemes takes advantage of the remaining available frequency spectrum for transmitting data and, therefore, can co-exist with POTS.
An xDSL modem is a device that modulates and demodulates signals across an xDSL physical interface. A transceiver unit at a remote location (xTU-R) refers to a modem located at a customer's site, and a transceiver unit at a central location (xTU-C) refers to modem located in a central office (CO) or remote terminal (RT) of a loop provider. Each transceiver typically includes a transmitter and a receiver. Again, the “x” refers generically to transceivers designed for different standards. For example, for ADSL the transceivers are referred to as an ATU-R and an ATU-C.
In the many standards of digital subscriber loops, various protocols including activation, initiation, training, and showtime have been designed for initializing communication with between the xTU-C and xTU-R. Activation, for example, is the process of discovery of the xTU-C by the xTU-R, or vice versa, through the use of protocol specific signals. For systems designed to operate with significant loop losses, crosstalk from xDSL systems on adjacent lines can cause significant problems, especially for activation signals. Crosstalk is a disturbance caused by an electric or magnetic fields of one telecommunication signal affecting a signal in an adjacent circuit.
Referring to FIG. 1, a block diagram illustrating a system affected by crosstalk is shown generally by numeral 100. The present example refers specifically to a non-overlapped ADSL system. That is, upstream and downstream transmit signals reside in separate, non-overlapped frequency bands. A first ADSL loop 102 connects a first ATU-C 106a with a first ATU-R 106b. A second ADSL loop 104 connects a second ATU-C 108a with a second ATU-R 108b. Typically, both the first and second ATU-Rs 106b and 108b are designed to be capable of responding to signals that may have experienced significant loop loss. This is true because they are designed to be able to operate on various loop lengths.
In the present example, the first loop 102 is longer than the second loop 104. The first loop 102 is relatively long, thus the two transmitters on that loop, the ATU-C 106a downstream transmitter as well as the ATU-R 106b upstream transmitter, transmit at full power so as to overcome those loop losses. The second loop 104 is relatively short (for example, having loop losses on the order of 10 dB or less). However, due to the proximity of the loops 102 and 104, as well as the proximity of the first ATU-C 106a and the second ATU-R 108b, there is significant crosstalk 110 coupling from the transmitter of the first ATU-C 106a to the receiver of the second ATU-R 108b. This crosstalk signal is referred to as Far End Crosstalk (FEXT), since the victim receiver (in the ATU-R 108b) is on the far end of the loop from the offending transmitter (in the ATU-C 106a). Generally, FEXT reduces as the distance increases between the victim and the offender. The crosstalk coupling loss can be on the order of 70 dB in the ADSL frequency band of interest, which is similar to the loop loss for a long loop. Therefore, the second ATU-R 108b may perceive the crosstalk signal from the transmitter of the first ATU-C 106a as a signal received from a distant ATU-C at the other end of its own loop, since the ATU-R 108b may not have a priori knowledge of the length of its own loop.
If the transmitted signal is an activation signal, the crosstalk can be falsely detected as a valid activation signal, especially where the crosstalk comes from an xDSL system of the same class. When activation signals are falsely detected, proper initialization of the transceiver falsely detecting the signal can be delayed, sometimes indefinitely.
Referring to FIG. 2, a graph illustrating a snapshot of the frequency spectrum in a non-overlapped spectra ADSL case is shown generally by numeral 200. The ATU-R 108b sends activation tones 202 in the upstream band, but the ATU-C 108a is not sending any tones in the downstream band. In this figure, the crosstalk signals 204 shown in the downstream band may cause the ATU-R 108b transceiver to become confused and attempt to activate the line.
Therefore, there is a need for a method of inhibiting a transceiver from responding to a crosstalk activation signal. It is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.